1. Field of the Invention
This invention relates generally to microphone systems, and, more specifically, to highly directional microphones providing a flat frequency response.
2. Background Information
In the reception and recording of sound, there are many applications when it is useful to have directional microphones. The standard technique is to rely on the directional response of microphone that is itself directional, such as a pressure gradient or “shotgun” type microphone. These microphones are limited both in the directionality of response and in the flatness frequency response. Various aspects of directional microphones of “classical” design are discussed in a number of articles, such as: Harry F. Olson “Directional Microphones,” Journal of the Audio Engineering Society, October 1967, and B. R. Beavers, R. Brown “Third-Order Gradient Microphone for Speech Reception” Journal of the Audio Engineering Society, December 1970. These two articles are included in “Microphones: An Anthology of Articles on Microphones from the Pages of the Journal of the Audio Engineering Society” Publications office of the Audio Engineering Society (1979), which is hereby incorporated by this references.
In a series of articles dating from the early 1970's, Michel Gerzon suggested using cancellation between two adjacent microphones to achieve high directionality in a limited frequency range. This is described in a series of articles: “Ultra-Directional Microphones: Applications of Blumlein Difference Technique: Part 1” Studio Sound, Volume 12, pp 434-437, October 1970; “Ultra-Directional Microphones: Applications of Blumlein Difference Technique: Part 2” Studio Sound, Volume 12, 501-504, November 1970; and “Ultra-Directional Microphones:
Applications of Blumlein Difference Technique: Part 3” Studio Sound, Volume 12, 539-543, December 1970, which are all hereby incorporated by reference. This is also similar to the techniques used in certain aspects of phased-array radar. By combining the output of the microphones, the interference between the outputs adds constructively in a direction perpendicular to the axis connecting the microphones, but cancels to a varying degree in other directions.
Although this results in a high degree of directionality to the response, it is highly dependent upon the relation between the microphones' spacing and the frequency of the sound. Although radar and other applications only require sensitivity in a fairly narrow frequency range, audio applications may require that the frequency response be flat over a sizable portion of the audio range.